The present invention relates to power transmissions, and in particular to a driving pulley for use in a continuously variable transmission.
A continuously variable transmission (CVT) is a mechanical device in which the torque transmission ratio is infinitely variable over the required working range, by contrast to a conventional transmission in which there is a limited number of selectable torque transmission ratios. A CVT automatically changes the ratio as required by load and speed conditions, providing an increased torque under high loads at low speeds and yet controlling the rotation speed of the motor as the vehicle accelerates. It is commonly used in a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts, all-terrain vehicles (ATV) and scooters. A CVT is used with internal combustion engines or electric motors.
A conventional CVT comprises a driving pulley connected to a motor, a driven pulley connected to wheels or a track, and a trapezoidal drivebelt transmitting torque between the driving pulley and the driven pulley. The sides of the drivebelt are, on each pulley, gripped between two opposite flanges that are to be coaxially mounted around a main shaft. One of the flanges is axially movable with reference to the other. Each pulley is directly or indirectly in a torque-transmitting engagement with the main shaft.
The driving pulley comprises a centrifugal mechanism that tends to increase the ratio when the rotation speed of the driving pulley increases. To do so, the centrifugal mechanism is able to apply a force on the movable flange to move it closer to the fixed flange. This will force the drivebelt to wind on a larger diameter on the driving pulley. At the same time, the change in the position of the drivebelt will cause a radial force to be exerted on the flanges of the driven pulley, in addition to the tangential driving force by which the torque is transmitted. This radial force urges the second flange of the driven pulley away from the first flange thereof.
Initially, when the vehicle is stopped, the winding diameter of the driving pulley is minimum and the winding diameter of the driven pulley is maximum. This is referred to as the minimum ratio since there is the minimum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley. Then, when the vehicle accelerates, the movable parts of the CVT will constantly seek to rearranged their position until an equilibrium is reached or until they reach the maximum ratio. The ratio at which the CVT stabilizes is an equilibrium between the forces on the drivebelt in the driving pulley and in the driven pulley.
At the maximum rotation speed, the ratio is maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
When the rotation speed of the motor decreases, the force generated by the centrifugal mechanism decreases. Return springs located in the driving and driven pulleys allow the corresponding movable flanges to move back to their original low ratio position.
The driven pulley of a CVT is also torque-sensitive. This allows the driven pulley to counterbalance the force generated by the centrifugal mechanism of the driving pulley so as to allow the motor speed to rise before the CVT starts upshifting during an acceleration from a low speed or to lower the ratio if proper conditions are met. This is caused by a cam system that tends to move the movable flange towards the fixed flange of the driven pulley when the torque increases, thereby pulling back on the drivebelt and fighting the opposite force from the driving pulley.
A conventional cam system of a driven pulley comprises a cam plate having a plurality of symmetrically-disposed inclined cam surfaces on which respective cam followers are engaged. The cam followers are generally slider buttons or rollers. The cam plate or the set of cam followers is mounted at the back side of the movable flange and the other of them is rigidly connected to the main shaft.
In a scooter, a conventional CVT further comprises a centrifugal clutch mounted next to the driven pulley. The centrifugal clutch is used to disengage the mechanical connection between the driven pulley and the output shaft when the scooter is stopped or if it moves too slowly. This allows the motor to at least operate at idle speed. The driving pulley, the driven pulley and the drivebelt are always rotating when the motor operates. Whenever the rotation speed of the motor increases, the rotation speed of the driven pulley increases as well and the clutch is moved to its engaged position under the effect of the centrifugal force that acts on small inwardly-biased flyweights. The flyweights are provided with friction pads. These pads eventually reach the interior side of a drum and force it to rotate as the friction engagement increases with an increase of the rotation speed. The friction pads and the drum remain connected as long as the rotation speed is above a threshold value.
The CVT of a scooter is generally located in a protective housing. This housing have the drawback of reducing the heat transfers between the CVT and the atmosphere. Yet, the centrifugal clutch is an important heat source. The temperature reached inside the housing during the operation of the CVT can be quite high, especially when the CVT is used in a hot environment. This may reduce the service life of the drivebelt since it generally decreases when the average operating temperature increases.
Unlike scooters and other vehicles, snowmobiles have successively used CVTs that are provided only with a driving pulley, a driven pulley and a drivebelt. The driving pulley is used as a clutch since the spacing between the two flanges thereof is allowed to be greater than the width of the drivebelt at low motor rotation speed. This corresponds to the disengaged position of the CVT and no force is then transmitted to the drivebelt from the motor, which is allowed to rotate freely at idle speed. As soon as the rotation speed of the motor increases, the flyweights inside the driving pulley come into action and force the movable flange to move toward the fixed flange. The drivebelt will eventually be gripped between the two flanges and the CVT will be in the engaged position. The drivebelt remains engaged in the driving pulley until the rotation speed is low enough so that the movable flange be moved out of engagement with the drivebelt under the action of the return spring.
In normal operating conditions, using the sides of a drivebelt as a friction element for the progressive engagement of the CVT does not significantly wear a good quality drivebelt. Recent improvements in the field of drivebelts lead to products that can have a long service life, in spite of the intense solicitation of the material. These drivebelts comprise Kevlar(copyright) or other synthetic materials. Moreover, the use of the drivebelt as the clutch is more efficient and generates less heat that the conventional centrifugal clutch with friction pads.
As aforesaid, a conventional driving pulley is usually provided with an helical spring to generate a return force. The return force generated by the spring is inversely proportional to the distance between the movable flange and the fixed flange. This is exactly the opposite of what it should be. Ideally, the return force should be maximum at the position of the movable flange where the drivebelt is disengaged and be proportional to the distance between the flanges of the driving pulley. A weaker return force at a higher ratio allows the driving pulley to respond more quickly and be more efficient.
It is the object of the present invention to provide a scooter with a CVT that uses the drivebelt as a clutch instead of a conventional centrifugal clutch with friction pads. One of the advantages of such arrangement is the reduction of the average temperature inside the housing of the CVT, which thus increases the service life of the drivebelt. It is also an object of the present invention to provide a driving pulley with a return mechanism that generates a return force proportional to the distance between the two flanges, more particularly a return force which decreases when the two flanges are moved closer to each other. More particularly, the present invention provides a driving pulley for a continuously variable transmission, the driving pulley being coaxially mountable around a drive shaft and rotatable at a variable rotation speed, the driving pulley comprising:
a first flange having one side provided with a conical wall;
a second flange having opposite first and second sides, the second flange being coaxial with the first flange and having a conical wall provided on the first side thereof, the conical wall facing the conical wall of the first flange to form a drivebelt-receiving groove around which a drivebelt is partially wound, the second flange being axially movable with reference to the first flange between a distal position and a proximal position;
a first means for connecting the first flange to the drive shaft in a torque-transmitting engagement;
a second means for connecting the second flange to the drive shaft in a torque-transmitting engagement;
a third means for generating a first force urging the second flange to move towards the first flange, the third means being responsive of the rotation speed of the driving pulley; and
a fourth means for generating a second force urging the second flange to move away from the first flange.
The driving pulley is characterized in that:
the fourth means generate the second force with a maximum magnitude when the second flange is substantially at the distal position and with a minimum magnitude when the second flange is substantially at the proximal position; and
the drivebelt-receiving groove is wider than the drivebelt when the second flange is at the distal position, whereby there is no contact between the conical wall of the second flange and a corresponding side of the drivebelt when the second flange is at the distal position.
According to another aspect of the present invention, there is provided a driving pulley for a continuously variable transmission, the driving pulley being coaxially mountable around a drive shaft and rotatable at a variable rotation speed, the driving pulley comprising:
a first flange having one side provided with a conical wall;
a second flange having opposite first and second sides, the second flange being coaxial with the first flange and having a conical wall provided on the first side thereof, the conical wall facing the conical wall of the first flange to form a drivebelt-receiving groove around which a drivebelt is partially wound, the second flange being axially movable with reference to the first flange between a distal position and a proximal position;
a first means for connecting the first flange to the drive shaft in a torque-transmitting engagement;
a second means for connecting the second flange to the drive shaft in a torque-transmitting engagement;
a set of flyweights to generate a first force urging the second flange to move towards the first flange, each symmetrically-disposed around the drive shaft between a corresponding pair of opposite and outwardly-converging ramps, one of the ramps of each pair being provided on the second side of the second flange and the other being provided on an interior side of an end plate facing the second side of the second flange and fixed with reference to the first flange; and
a fourth means for generating a second force urging the second flange to move away from the first flange.
The driving pulley is characterized in that it further comprises at least one torsion spring having at least one coiled portion and a lever arm, the lever arm being in a sliding engagement with a corresponding convexly curved ramp, the coiled portion being connected to one among the second flange and the end plate, and the convexly curved ramp being provided on the other of them.